True altitude variation meter



March 4, 1958 y H. J. SANDBERG I 2,825,227

TRUE ALTITUDE VARIATION METERv i Filed Apri1'9, 1954 {lllmlllllll FUTURA/EY TRUE ALTITUDE VARIATION? IVETER Herbert J. Sandberg, NewYork;LNY.;assignorgbyinesne assignments, to Norden-Keta'yCorporation, a corporation of Illinois l Appiic'aiiniaprn w19-setserial N maar# s claimsifftclsvs-ss) My inventionlfrelates. qto' LAat true? altitudewvariatioir` meter and more@ particularly to? a :barometric altimeterrrwlii'ch is-automatically correctedfforferrorsewhich would'fbe in troduced by actual changes in temperature: f

Barometric altimeters ofv the prior art are arranged to read altitude as a function-i-ofwbarometric pressure. As

an airplane rises, thedensit'y of the circurnambient air, 'Y pr'ssu and hence the barometric pressure, decreases as a fui tion ot the change.iinfaltitucle.J An altitude-indicating device may, therefore; bercalibratedlas a function of barometric pressure. However, density of theair is not tenipf changes? -As a fresult'; the barorrietric""pre'sf sure is alected by changes in temperature. Consequently; temperature changes introduce errors into altitude-indicating devices which-arecalibrate'd directly in proportidn to changes in barometricpressurew In4 order to correct for such errors, the prior art relied nme'clianis'mjfwhich correct for temperature change in accordance witha pressure-temperature relationship based on a standard atmosphere. Since the actual temperature and actual tempera-r ture variation will rarely be, thesame as the standard atmosphere upon whihth'e'-pressurelternperature relationshipfjwlas' based', i errors 'will 'be introduced.. s

I.,have"inve"1`1ted a truev altitude variation-meterzwhich autintically'correctsfor errorsv which would be introd'fce'cl'-l`xy' actual changes in` temperature.l My altimeter lgiyesltruefaltitiide reading-s-fwithoutthe necessity otre sortiiigtoa mechanism based onta standard-atmosphere. Myfiiieter is also arranged to providefair density readings ina manner which willv beA described more fully he'reinafterT y One .objctof invention is to provide a-'truet altitude variation meter -in-which correction is made automatically ice :heap essere responsive devicefto a pssiadn of baume.

multiplier.: Tlesec'ondfarm "ofmy" bridge is responsive toi-.hange's ii'nf temjgicrature.v Tlie="thir'd armi vof'myV bridge' isvariedinfccordnce'fwith a function: fMachs number' so'th'a't:thebridgeineasures th'e'de'nsity 'of thef'cireui'ain bi'e'ntv latmosphere, fini a manner'l which will V be described' indetailhereinafter. I provide'mansactatedbya signalV pro'portion-alf! to'lthe bridge; unbalance' 'for 'varying the bridge`r balancing; A 'arm in' acc-:ordan'c'e with" density Y to'Y balance the bridge.-Tl1'isfmean's; actuated by `a ysignal pro# `oprftionalf.tolliridgeimbalance,fisfalso 'adapted to positionttheib soflthej ball' f and disk multiplier.:

Itf-isla'w'p 'fsical'fact "thatv` an 'increme'ntal fchavngel in @equal tolthe" density 'mltipliedfby' a corres p'ondingfinerere'ntalf @harige-inftru'e altitude; Since the' outputfof` fmyl i-rnul-tiplier'lf is `proportional to: fchaingesfin pressuref-andh balls ofytheiiirltiplier arefpo'si'tioned in accordance withffdeiii ygfthe inpiit tox the-multiplier disk must `be`y proportional-*t'oith'e corresponding change "in'v true altitudel Consequently; the means sponsive'to-chaniges infpositioir,` off'fhe'fpressureresponsiv device'inust drive thee;- input?` `'shaft of/f' the' i multiplier in@j aclcbrd'zt'nc'efy with changes in true altitude. A Asuitable metering devicemay variati/ont indications. The -`lini al l altitude may ybe `*set into'fthe de'vi'cel-b'yanyconvenintmefans; to2 provide la true altimet'er': 1 It'fwill" be 'shown that since orieifofy ther` arms true altitude variationmeter,whichvis adaptdeto measure ,airffdejnsityr y Otherfand --further-*objects of ymy -invention will ,appear from the followingldescription.1f

Ins-,generaL my f invention contemplates'fthe'r provision offra device responsive torchanges in=barometricipressure and means lfor? balancing-'there pressure exerted by fthe-L tmosphere'on thewpress'urerresponsiveIl device. "f I provide meansfresp'onsive ftorlchnges in r thef positionf' ofjthelpre'sl sureresponsiveVde-viceeforfdriviiig theldiskbf-"f ball? and nectfonefend'offthe spring-i 22-infa hole"'26' in linkv14 and th'efother' end-'toy a lug 24- atthe bottom of Athe in'- terio'rofa hollow'rack20, yslidably mounted ina housing 302i/ Rack. Ztll'has :teeth Ztl/formed on`r oney side thereofV andfis positioned by va gear 32 which engages', teeth 2S. Gear32`is driven by a shaft 34 to position the rack ztof'ftensinithe spring 22'to`balance the'forceexerted polarity corresponding to the direction of movement of arm 36.

I connect the respective output terminals 64 and 66 of amplifier 54 to a motort68 by conductors 70 and 72 and complete the motor circuit with a ground connection 74. As the contactor 44 of arm 36 engages contact 46 or contact 48, motor 68 rotates in one direction or the other. The output shaft 76 of motor 68 drives gearing 78, 82, 80, which drives the input shaft 84 of a ball and disk multiplier including a disk 86 and balls 88. The balls 88 are carried by a cage 90 which is adapted to be moved radially of the disk 86 in accordance with density, as will be explained hereinafter. .Theoutput roller 94 of the multiplier drives the shaft 34` through gears 96, 98, in accordance with changes in pressure to tension spring 22 to balance the force resulting from the pressure exerted by the/atmosphere on bellows l0. y

As long as the force exerted by the spring'22 is not equal to the force resulting from the pressure of the atmosphere on bellows 10. the contactor 44 of arm 36 continues to engage one of the contacts 46 or 48 of the amplifier 54 `and the amplifier continuously produces an output signal. The polarity of this signal is such that motor 68 and consequentlyshaft 34 are driven in one direction or the other until spring 22 exerts a force on bellows `which is equal and opposite to the force resulting from the pressure of the atmosphere. The rotation of shaft34 adjusts the tension on spring 22 by moving the rack member up or down with respect to housing 30.

The direction of rotation of shaft 34 is such that changes in spring tension balance the pressure of zthe atmosphere on the bellows 10. The position .of balls 88 is a function of density. Accordingly, the rotation of the input shaft 84 is such that the amount of rotation of shaft 34 is proportional to changes in pressure. The balls 88 are positioned by means energized from the output signal of a Wheatstone bridge which drives arm 160 to measure density. This bridge includes a first ratio arm 100, a second ratio arm.102, a third arm 104, and` a balancing arm 106;` A suitable source of electrical energy such as a battery 108 is connected across one pair of terminals 110 and 112 of the bridge. A brush 114, associated with the ratio arm 100, is driven by a shaft 116, connected to the shaft 34 `by gearing 118, 120. Brush 114 is thereby positioned to change the resistance of arm 100 in accordance with changes in pres sure. Arm 102 is a temperature probe for measuring temperature. A brush 122, associated with the third arm of the bridge, is positioned by a shaft 124, driven by a shaft 130 through gears 126, 128. Shaft 130 is actuated by a Mach number meter 132 of a type well known in the art. For example, a Mach number meter such as is disclosed in the copending application of William C. Coulbourn and Herbert I. Sandberg, Serial No. 335,# 542, filed February 6, 1953, now Patent No. 2,694,927, dated November 11, 1954, may be employed.

In, order to provide a means for balancing my bridge, I connect an amplifier 134 between the terminals 136 and 138. This amplifier produces an output signal which is proportional to the amount of unbalance of the bridge. A pair of conductors 140 and 142 feed this output signal to a motor 144. The shaft 146 of motor 144 has a bevel gear 148 thereon which drives a pair of bevel gears 150 and 152. Gears 150 and 152 engage a gear 154 fixed on one end of a shaft 156. Gear152 drives a shaft 158 which actuates'the brush 160 corresponding to the balancing` 4 Let:

R100, R102, R104, and Rm=the respective `resistances of bridge arms 100, 102, 104, and 106 P=pressure of air t l T =measured temperature of arr brought rsentropically to rest Tur-temperature of the free air p=density of the atmosphere specific heat of air at constant pressure "specific heat of air at constant volume speed of craft *velocity of sound in air at T0 At balance, the relationship of my bridge is:

p R104 (2) T-toszKMz Z (3) Rmgpuisnn The relation between free air temperature To and the measured temperature T, `air brought isentropically to rest, is:

. T y (4) T"1+o.2r Mz Substituting for T in Equation 3, I obtain: 5 Rm==cp where C isa proportionality constant.

From the foregoing it will be apparent that shaft 146 rotates in accordance with the density of the atmosphere to rotate shaft 158 through gears 148, 152 to drive brush 160 and vary the resistance Vof arm 106 to balance the bridge. Accordingly, shaft 156 is positioned in accordance with density. Shaft156 drives the lead screw 92 through bevel gears 162 and 164 to position the cage 90 carrying the balls 880i the multiplier in accordance with density.

It has been demonstrated that in order to balance the force resultingA from the pressure of theatmosphere on the bellows 10, the output roller 94 of the ball and disk multiplier must be drivenin accordance with changes in pressure. Since the balls 88 of the multiplier are radially positionedinaccordance with density, it follows from the physical relation dP=pdh that the input shaft 84 of the multiplier must be rotated in accordance with changes in altitude at balance where dh is a change in altitude and dP is the corresponding change in pressure. A shaft 166,'"fixed on the bevel gear 82, provides a means for actuating the pointer 168 which `cooperates with a scale 170, suitablycalibrated to indicate altitude changes. If the scale 170 is to read true altitude values, rather than `merely changes in true altitude, `the initial altitude may be set into the instrument by any appropriate means.

In order to indicate density, I actuate the pointer 174 'associated with a suitably calibrated scale 176 by means of a shaft 172 driven by the gear 150.

In operation, the initial altitude may be set on the scale 170 by any .means known to the art, such` as a differential or the like. Assuming that the aircraft rises so that the barometric pressure decreases, spring 22 expands bellows 10 and arm 36 pivots about `the pivot 38 so-that its contactor 44 engages amplifier contact` 46. This completes the 'circuit of the amplifier 54 `to produce maneges? an output signal fdrene'rgizidgr-thmotbr 68 to drive the vs ln'tft76;andcrotate disk 86 through.,g ears 78, 2randsr80. This rotation of disk 86 is in'a'direction to mo've the rack 20 upwardly,;totredt'mtxhe2 tensionion spring 22 until the force exerted on bellowsi bythe atmosphere is balanc'edf' Rotation' "of l"the'disk86continuesumlbalanee isireach'd""sinc'e','-=in the unblaieettfeonditonfa Ysignal-s continuously produced by ampli'r'54." Sicetlfsys'tem automatically comes to balance, the amount of rotation of shaft 34 must be proportional to changes in pressure.

As has been explained hereinbefore, the Wheatstone bridge including arms 100, 102, 104 and 106 measures density. Shaft 156 will, therefore, be positioned by motor 144 in accordance with the density as the aircraft rises to position balls 88 radially along the face of disk 86 as a function of density. Since the output of the multiplier must be proportional to changes in pressure and the balls 88 are positioned according to density, it will readily be seen that the multiplier solves the equation:

(6) dP==pdh From the above equation, knowingrthe output of the multi` plier, dP, and the position of the balls, p, it is readily apparent that the input to the multiplier, or the amount of rotation of the shaft 76, must be dh.

If I originally set the initial altitude into the system, scale 170 can be calibrated to indicate the true altitude value. Since the output of the Wheatstone bridge, represented by the rotation of the shaft 146, is proportional to density, scale 176 indicates density. Y

If the aircraft descends so that the barometric pressure increases, contactor 44 engages contact 48 and shaft 76 rotates as a function of the decrease in altitude. As before, this indication of a change in altitude is a true indication, since correction for changes in actual temperature is automatically made in the Wheatstone bridge as it measures density.

While I have described my instrument as a meter for measuring absolute altitude, it is primarily intended to function as a true altitude variation meter. In order that the meter read absolute values of altitude, the device must be set to the initial value of altitude.

Thus it will be seen that I have accomplished the objects of my invention. I have provided a true altitude variation meter which is automatically corrected for errors which would be introduced by changes in actual temperature. My meter is adapted also to measure density and corrects the altitude indication for variations in density resulting from temperature changes. My true altitude variation meter eliminates the necessity of resorting to a mechanism which operates in accordance with a pressuretemperature relationship based on a standard atmosphere to obtain an altitude indication. physically correct relationship dP=pdh rather than an assumed relationship based on a standard atmosphere.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of my claims. It is further obvious that various changes may be made in details within the scope of my claims without departing from the spirit of my invention. It is therefore to be understood that my invention is not to be limited to the specific details shown and described.

Having thus described my invention, what I claim is:

1. A true altitude `variation meter including in combination a barometricV pressure responsive device, sensing means 'actuated by said pressure responsive device, a multiplier having a pair of input channels and an output channel, means responsive to a change in position of said sensing means for feeding a first one of said input channels, means for producing a signal which is 'a function of the density of the atmosphere, means energized by a change in said signal for feeding the second of Y said multiplier input channels, means operated by the outon said pressure responsive tdevice' wherebyzfsaielz',meansv.l

responsive to the position of the sensing means produces a responseiwhicl iis affunctiontlofrchanges:inialtitude, means operated-=by fsaidumultiplierroutput channel for feeding the multiplier output to said density signal pno ducinglm'e'ans, "and 'indicating' 'means' "connected 'to-alid actuated by'said 'inea'n's" responsive'to' the-` position' "of sa'id sensing means.

2. A true altitude variation meter including in combination a barometric pressure responsive device, sensing means connected to said pressure responsive device, a multiplier including a disk, balls and an output roller, said balls Ibeing positioned to provide a driving connection between said disk .and` said output roller, means responsive to a change in position of said sensing means to rotate said disk in a direction corresponding to said change in position, means for producing a signal which is a function of the density of the atmosphere, means responsive to said signal to position the balls of said multiplier an amount representative of the magnitude of the density signal, means operated by said multiplier out put roller for balancing the pressure on said pressure responsive device whereby said multiplier diskV is rotated in accordance with changes in .altitude and indicating means connected to and actuated by said means responsive to the sensing means.

3. A true altitude variation meter asin claim 2 wherein said means for producing said signal is a Wheatstone bridge including three variable resistance arms and a temperature responsive arm, the resistance of a first one of said variable resistance arms being varied by said means operated by the multiplier -output shaft, means for varying the resistance of a second one of said variable r resistance arms in accordance with a function of Machs number and means responsive to unbalance of the Wheatstone bridge for producing said signal, said means responsive to said signal varying the resistance of the third one of Said variable resistance arms to bring the bridge to balance.

4. A true altitude variation meter including in combination a bellows adapted to expand and contract in response to changes in atmospheric pressure, a spring attached at one end to the bellows, means for tensioning My meter solves the said spring to exert a force to balance the pressure exerted on the bellows by the atmosphere, sensing means connected to said bellows, a multiplier having a pair of t input channels and an output channel, means responsive to a change in position of said sensing means for feeding a rst one of said input channels, means for producing a signal which is a function of the density of the atmosphere, means energized by a change in said signal for feeding the second of said multiplier input channels, the output channel of said multiplier actuating said spring tensioning means to balance the pressure exerted on said bellows, the arrangement being such that the means responsive to the position of said sensing means produces an output which is proportional to changes in altitude, means operated by said multiplier output channel for feeding the multiplier output to said density signal producing means, and indicating means actuated by said means responsive to the position of the sensing means.

5. A density meter for measuring the density of the atmo-sphere comprising in combination la Wheatstone bridge having three variable resistance arms and a fourth resistance arm responsive to changes in temperature of the atmosphere, means for varying the resistance of a Y first of said variable resistance arms in accordance with 7 8 to balance the bridge and ndiating `means actuat .d by 1 FOREIGN PATENTS Sad mmseefgzdby adsinal i 722,062 Germany Jum 29, 1942 References` Citedin the file of this patent n OTHER REFERENCES UNITED STATESPATENTS 5 Zublz: Analysis and Design of Translator Chains," 2,412,541 Shivers Dec. 10,` 1946 v01. L Askana Regulator Co., Chicago, Illinois, 1946.

2,557,092 Garbarive June 19', 1951 Pages 209 and 210 `relied on. 

